Despite the tremendous progress in molecular biology and the identification of genes, mutations, and polymorphisms responsible for disease, the ability to rapidly screen a subject for the presence of multiple disorders has been technically difficult and cost prohibitive. Current DNA-based diagnostics allow for the identification of a single mutation or polymorphism or gene per analysis. Although high-throughput methods and gene chip technology have enabled the ability to screen multiple samples or multiple loci within the same sample, these approaches require several independent reactions, which increases the time required to process clinical samples and drastically increases the cost. Further, because of time and expense, conventional diagnostic approaches focus on the identification of the presence of DNA fragments that are associated with a high frequency of mutation, leaving out analysis of other loci that may be critical to diagnose a disease. The need for a better way to diagnose genetic disease is manifest.
With the advent of multiplex Polymerase Chain Reaction (PCR), the ability to use multiple primer sets to generate multiple extension products from a single gene is at hand. By hybridizing isolated DNA with multiple sets of primers that flank loci of interest on a single gene, it is possible to generate a plurality of extension products in a single PCR reaction corresponding to fragments of the gene. As the number of primers increases, however, the complexity of the reaction increases and the ability to resolve the extension products using conventional techniques fails. Further, since many diseases are caused by changes of a single nucleotide, the rapid detection of the presence or absence of these mutations or polymorphisms is frustrated by the fact that the PCR products that indicate both the diseased and non-diseased state are of the same size.
Developments in gel electrophoresis and high performance liquid chromatography (HPLC), however, have enabled the separation of double-stranded DNAs based upon differences in their melting behaviors, which has allowed investigators to resolve DNA fragments having a single mutation or single polymorphism. Techniques such as temporal temperature gradient gel electrophoresis (TTGE) and denaturing high performance liquid chromatography (DHPLC) have been used to screen for small changes or point mutations in DNA fragments.
The separation principle of TTGE, for example, is based on the melting behavior of DNA molecules. In a denaturing polyacrylamide gel, double-stranded DNA is subject to conditions that will cause it to melt in discrete segments called “melting domains.” The melting temperature Tm of these domains is sequence-specific. When the Tm of the lowest melting domain is reached, the DNA will become partially melted, creating branched molecules. Partial melting of the DNA reduces its mobility in a polyacrylamide gel.
Since the Tm of a particular melting domain is sequence-specific, the presence of a mutation or polymorphism will alter the melting profile of that DNA in comparison to the wild-type or non-polymorphic DNA. That is, a heteroduplex DNA consisting of a wild-type or non-polymorphic strand annealed to mutant or polymorphic strand, will melt at a lower temperature than a homoduplex DNA strand consisting of two wild-type or non-polymorphic strands. Accordingly, the DNA containing the mutation or polymorphism will have a different mobility compared to the wild-type or non-polymorphic DNA. The TTGE approach has been used as a method for screening for mutations in the cystic fibrosis gene, for example. (Bio-Rad U.S./E.G. Bulletin 2103, herein expressly incorporated by reference in its entirety).
Similarly, the separation principle of DHPLC is based on the melting or denaturing behavior of DNA molecules. As the use and understanding of HPLC developed, it became apparent that when HPLC analyses were carried out at a partially denaturing temperature, i.e., a temperature sufficient to denature a heteroduplex at the site of base pair mismatch, homoduplexes could be separated from heteroduplexes having the same base pair length. (See e.g., Hayward-Lester, et al., Genome Research 5:494 (1995); Underhill, et al., Proc. Natl. Acad. Sci. USA 93:193 (1996); Oefner, et al., DHPLC Workshop, Stanford University, Palo Alto, Calif., (Mar. 17, 1997); Underhill, et al., Genome Research 7:996 (1997); Liu, et al., Nucleic Acid Res., 26:1396 (1998), all of which and the references contained therein are hereby expressly incorporated by reference in their entireties). Techniques such as Matched Ion Polynucleotide Chromatography (MIPC) and Denaturing Matched Ion Polynucleotide Chromatography (DMIPC) have also been employed to increase the sensitivity of detection. It was soon realized that DHPLC, which for the purposes of this disclosure includes but is not limited to, MIPC, DMIPC, and ion-pair reverse phase high-performance liquid chromatography, could be used to separate heteroduplexes from homoduplexes that differed by as little as one base pair. Various DHPLC techniques have been described in U.S. Pat. Nos. 5,795,976; 5,585,236; 6,024,878; 6,210,885; Huber, et al., Chromatographia 37:653 (1993); Huber, et al., Anal. Biochem. 212:351 (1993); Huber, et al., Anal. Chem. 67:578 (1995); O'Donovan et al., Genomics 52:44 (1998), Am J Hum Genet. December;67(6):1428-36 (2000); Ann Hum Genet. September:63 (Pt 5):383-91 (1999); Biotechniques, April;28(4):740-5 (2000); Biotechniques. November;29(5):1084-90, 1092 (2000); Clin Chem. August;45(8 Pt 1):1133-40 (1999); Clin Chem. April;47(4):635-44 (2001); Genomics. August 15;52(1):44-9 (1998); Genomics. March 15;56(3):247-53 (1999); Genet Test. ;1(4):237-42 (1997-98); Genet Test.:4(2):125-9 (2000); Hum Genet. June;106(6):663-8(2000); Hum Genet. November;107(5):483-7 (2000); Hum Genet. November;107(5):488-93 (2000); Hum Mutat. December;16(6):518-26 (2000); Hum Mutat. 15(6):556-64 (2000); Hum Mutat. March;17(3):210-9 (2001); J Biochem Biophys Methods. November 20;46(1-2):83-93 (2000); J Biochem Biophys Methods. January 30;47(1-2):5-19 (2001); Mutat Res. November 29;430(1):13-21(1999); Nucleic Acids Res. March 1;28(5):E13 (2000); and Nucleic Acids Res. October 15;28(20):E89 (2000), all of which, including the references contained therein, are hereby expressly incorporated by reference in their entireties. Despite the efforts of many, there remains a need for a better approach to screen for mutations and/or polymorphisms.